Part 5 - Design, Calibration & Performance of Standing-Wave-Ratio Meters - Calibration ProcedureAuthor: R.J.Edwards G4FGQ © 28th October 2000
Program Notes (Continued)
Connect the meter between a transmitter and a dummy load = Zo ohms. Switch the transformer secondary winding to the 'reflected' position. At medium trasmitter output power, at a low frequency, vary the potentiometer K for a voltmeter null. The ratio K is now equal to R3/Zo/N and the RF voltage available to drive the meter full scale for forward power is 2K times the load voltage at full rated transmitter power. Do not adjust K again - that part of the calibration procedure is complete.
Now reverse transformer connections to the 'forward' position. Alternatively reverse direction of transmission through the meter which will now indicate a large value on the forward power scale. Drive transmitter power output to a known value, say 100 watts into Zo ohms and adjust the voltmeter calibration resistor for full scale = 100 watts, or another preferred meter deflection bearing in mind that some loads may result in an indicated forward power greater than 100 watts. Both reflected and forward power, in watts, can now use this common scale and transmitter output power = indicated forward power minus indicated reflected power.
When calibrating the power scale, e.g., full scale = 100 watts, the 'known' power level should be checked by measuring the RF voltage across load Zo.
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